光科系学术交流报告

Direct Generation of Polarization Entangled Photons in Fiber

 Prof. Li Qian

Department of Electrical and Computer Engineering,

University of Toronto

Entanglement, a quintessential concept of quantum mechanics, is now an extremely powerful resource for quantum technologies. Once described by Einstein as the “spooky action at a distance”, the unique properties of entangled photons have been exploited in a range of applications, include quantum key distribution, state teleportation, and enhanced optical metrology. The generation of entangled photons was first demonstrated in a bulk BBO crystal through a second-order nonlinear process of spontaneous parametric downconversion (SPDC). However, such bulk free-space configuration suffers from poor mode definition and high coupling loss into fiber or other waveguide devices. Recent efforts have been shifted to developing waveguide sources of entangled photons. In particular, fiber-based sources are extremely attractive because of their compatibility with fiber lasers and fiber-optic telecom systems. However, due to the lack of second-order nonlinearity in conventional optical fibers, correlated photon pairs have been generated through spontaneous four-wave mixing (sFWM), a third-order process. There are several disadvantages with using sFWM, the biggest one being the contamination of Raman photons, which can only be suppressed by cryogenic cooling of the fiber. Another disadvantage is that the photons generated are not polarization entangled, and additional steps (such as a Sagnac loop and the removal of which-way information) are required to "convert" the correlated photon pairs into polarization-entangled photons.

In contrast, I will present a straightforward approach to generate polarization entangled photons directly in fiber. We utilize the SPDC process a poled fiber that exhibits an effective second-order nonlinearity, and we design the fiber to be weakly birefringent such that we can spectrally separate the type-II process for direct generation of polarization entanglement. The generated photons are in telecom-band, and can be easily separated with a C/L splitter (one in the C-band and one in L-band). We have achieved coincidence-to-accidental ratio CAR of >80:1, and we demonstrate high-visibility (>95%) two-photon interference in two non-orthogonal bases, and observed the violation of Bell's inequality by 18 standard deviations.

Brief Bio:

Dr. Li Qian is an associate professor in the Department of Electrical and Computer Engineering, University of Toronto. She brings both industrial and academic expertise to fiber-optic technologies. In 2001, she led the optical design of the world's first commercial extended L-band erbium-doped fibre amplifiers in Corning Inc. After returning to academia, she continued her pursuit in novel fibre-optic devices and systems, and in commercializable technologies. Her research group is currently conducting experimental research in quantum key distribution, fiber sensor interrogation, and nonlinear fiber devices, including fiber-based entangled photon sources. Her group's research on dispersion measurement was commercialized by Inometrix Inc., a start-up company headed by one of her graduate students. Prof. Qian is a senior member of the IEEE, and has served on a number of technical committees of OSA topical meetings. Prof. Qian holds a Canada Research Chair in Photonic Technologies and Applications.

时间：11月23日 (星期三) 下午2:00

地点：光学楼 525